设等差数列｛an｝的公差为d不为0,a1＝9d,若ak是a1与a2k的等比中项,则k＝_其他解答

设等差数列｛an｝的公差为d不为0,a1＝9d,若ak是a1与a2k的等比中项,则k＝

人气：262 ℃　时间：2019-08-19 18:16:19

解答

a1=9d
ak=a1+(k-1)*d=9d+(k-1)*d
a2k=a1+(2k-1)*d=9d+(2k-1)*d
ak^2=a1*a2k
化简后可求出k=4